Method and system for adjusting a curvature of a load plate based on a target load

ABSTRACT

A method of making a load plate for an attach hardware assembly of a circuit assembly includes adjusting a curvature of the load plate based on a target load to be applied by the attach hardware.

BACKGROUND

[0001] An Application-Specific Integrated Circuit (ASIC) is a microchip designed for a special application. The ASIC is designed to process information or complete tasks in a manner specific to the intended application. For example, ASICs are used in such diverse applications as auto emission control, environmental monitoring, and personal digital assistants (PDAs). ASICs are contrasted with general integrated circuits that can be used to perform different tasks for different applications. Examples of general integrated circuits include the microprocessor and the random access memory chips in a typical personal computer.

[0002] An ASIC can be mass-produced for a special application or can be custom manufactured for a particular customer application. Custom production is typically performed using components from a “building block” library of ASIC components. Each ASIC includes a number of input/output (I/O) leads that allow the ASIC to be connected to a larger circuit and receive the signals and data with which the ASIC works. These I/O leads are typically arranged in an array known as a Land Grid Array (LGA). The ASIC is usually attached to a circuit board, such as a printed circuit board (PCB). Leads or a socket on the circuit board make contact with the I/O leads of the LGA and connect the ASIC to the larger circuit of which it is a part.

[0003] The ever growing I/O count in today's large ASICs requires a very high clamping load to secure the ASIC to the circuit board and ensure continuous electrical contact between the ASIC and the circuit on the PCB. Clamping loads in the range of 400 to 700 pounds are becoming common. As noted, a socket may be provided on the PCB into which the ASIC is clamped.

[0004] The load necessary to secure the ASIC to the PCB is produced by the hardware used to attach the ASIC to the circuit board. This hardware is frequently referred to as the “attach hardware.” The attach hardware includes a bolster plate and a load plate. The load plate is a rigid plate that is typically made of steel and is sometimes referred to as a spring plate. The bolster plate is similar.

[0005] The ASIC assembly is sandwiched between the load plate and the bolster plate. Load studs connect the load plate and bolster plate, and a load screw is tightened to push the load plate and bolster plate apart causing the load plate to flex and generate the desired clamping force to the ASIC and circuit board.

[0006] Because the clamping load required is so high, the force can cause the PCB and/or the bolster plate to bow or deflect. This will impede the operation and performance of the socket or other connection between the ASIC and the PCB. Consequently, the bow of the bolster plate must be minimized. In some applications, this requires minimizing the load applied. On the other hand, the socket or connection between the ASIC and PCB requires a certain minimum load to be reliable. The result is that the load is constrained from above and below. The load must be sufficient to provide a reliable connection in the socket or between the ASIC and PCB, but must not be strong enough to cause a significant bow in the bolster plate or circuit board.

SUMMARY

[0007] A method of making a load plate for an attach hardware assembly of a circuit assembly includes adjusting a curvature of the load plate based on a target load to be applied by the attach hardware.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The accompanying drawings illustrate various embodiments of the present invention and are a part of the specification. The illustrated embodiments are merely examples of the present invention and do not limit the scope of the invention.

[0009]FIG. 1 is an exploded view of an ASIC assembly including an attach hardware assembly designed to provide a desired load according to an embodiment of the present invention.

[0010]FIG. 2 is an illustration of the ASIC assembly of FIG. 1 when assembled.

[0011]FIG. 3a illustrates a load plate at a starting, uncompressed shape.

[0012]FIG. 3b illustrates a load plate at a deflected, working shape.

[0013]FIG. 4 is a flowchart illustrating a method of controlling the load applied by an attach hardware assembly according to another embodiment of the present invention.

[0014] Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.

DETAILED DESCRIPTION

[0015] The present specification describes a method and system for controlling the curvature of the load plate in the attach hardware of an ASIC assembly so as to produce the desired target load at the target deflection when the attach hardware is in place. The curvature and, consequently, the spring rate of the load plate are chosen so that the deflection of the load plate when put in service produces the target load.

[0016]FIG. 1 illustrates an ASIC assembly (100) including an attach hardware assembly. As shown in FIG. 1, a typical ASIC assembly includes an ASIC (106) that is electrically connected to a circuit board (107), for example a printed circuit board. Typically, the ASIC (106) is connected to the circuit board (107) using a socket (105). An insulator (108) is disposed below the circuit board (107) to insulate the circuit board (107) from the attach hardware assembly which will be described below.

[0017] A heat sink (104) is typically included in the assembly to dissipate heat generated by the ASIC (106). An ASIC (106) will generate heat as it operates and, if this heat is not dissipated, can cause damage to the ASIC (106) or other components of the assembly (100). An Electro Magnetic Interference (EMI) gasket ( 111) and EMI frame (112) are also positioned between the heat sink (104) and the circuit board (107). A thermstrate (113) is positioned between the heat sink (104) and the ASIC (106) to enhance the thermal interface and improve thermal conduction from the ASIC (106) to the heat sing (104).

[0018] As described above, the ASIC (106) and circuit board (107) are held together with an assembly of attach hardware. Among other things, the attach hardware includes a load plate (101) and a bolster plate (109). Load studs (103) are also part of the attach hardware and run between the load plate (101) and the bolster plate (109).

[0019] As shown in FIG. 1, the load plate (101) has a curvature, for example, the load plate (101) has a cylindrical curvature. This curvature of the load plate (101) allows the load plate (101) to act as a spring, with a resulting spring rate, to apply a load to the ASIC assembly (100).

[0020] Another component of the attach hardware is the load screw (102). The load screw (102) is designed to cause a deflection of the load plate (101) that decreases the curvature of the load plate (101) and causes the load plate (101) to apply the target load to the assembly (100). The load screw (102) is screwed through a threaded hole in the load plate (101). The load screw (102) is driven through the load plate (101) until the end of the load screw (102) contacts the heat sink (104). Continuing the drive the load screw (102) against the heat sink (104) will cause the load screw (102) to applying an upward force on,the load plate (101) that deflects the load plate (101) reducing the curvature of the load plate (101). The corners of the load plate (101) are held in place relative to the assembly (100) by the load studs (103). Thus, the center of the load plate (101) is deflected upward against the curvature of the load plate (101). Consequently, the load plate (101) applies a load to the assembly (100) that is determined by the spring rate and deflection of the load plate (101).

[0021] The circuit board (107), the socket (105), the ASIC (106) and the thermal interface material (113) are sandwiched between the bolster plate (109) and the heat sink (104) under the load created by deflection of the load plate (101). As shown in FIG. 1, the heat sink (104), ASIC (106), socket (105), circuit board (107) and insulator (108) are all sandwiched between the load plate (101) and the bolster plate (109) to complete the ASIC assembly (100).

[0022] Assembly of the ASIC assembly (100) is performed as follows. The bolster plate (109) and EMI Frame (112) are attached to the circuit board (107) via the load studs (103). The socket (105) and ASIC (106) are placed onto the circuit board (107). Then, the heat sink (104) is lowered down onto the ASIC (106) over the load studs (103). The load plate assembly (101, 102) is shuttled onto the load studs (103) and the load screw (102) is driven through the load plate (101) to contact the heat sink (104). The assembled unit is illustrated in FIG. 2. However, in FIG. 2, the load screw (102) is not fully inserted. The load screw (102) is tightened until the head of the load screw (102) is brought into contact with the load plate (101) or a washer and applies pressure to deflect the load plate (101), which deflects the load plate to the target deflection.

[0023] This specification disclosed a process by which spring rate variation in the load plate (101) is significantly reduced by geometric compensation. As will be appreciated, if the spring rate of the load plate (101) varies during the manufacture of ASIC assemblies, the load applied in any given assembly will also vary accordingly. This may lead to inconsistent product reliability and/or failure of specific assemblies that experience a load outside the acceptable working range.

[0024] As described above, the load in the ASIC assembly comes from compressing or deflecting a cylindrically curved load plate from a starting height to a specific compressed height. FIG. 3a illustrates a load plate (101) with a starting height or curvature (S) which is the shape of the load plate (101) after production and with no force exerted on the plate (101) by a load screw. FIG. 3b illustrates a load plate (101) that is in service and deflected to a working height (D), which is less than (S). The load plate (100) will naturally resist this deflection and apply the desired load to the assembly.

[0025] To deliver the high loads required and survive the resulting stress, the load plate is often fabricated out of stainless steel and then heat-treated and tempered to achieve very high yield stresses. After forming and heat-treating, the load plate is compressed to just beyond the target deflection to eliminate first-cycle plastic yield. This results in a part that can be reused with the consistent results. This is referred to as sizing the part.

[0026] Ideally, each load plate produced through this process will follow the same force deflection curve. However, in reality, the load produced by an individual load plate in use is a function of the change in height or the deflection of the plate from a free state to an installed state multiplied by the spring rate of the plate at that working height. The spring rate is a function of the geometry of the load plate (e.g. the thickness of the plate or plates used to form the load plate) and the material from which the load plate is formed.

[0027] As noted above, it is desirable to minimize variation in the load applied by the load plates in different ASIC assemblies to promote consistency and product reliability. Since the installed or deflected height of a load plate is constant, the variation in the resulting applied load arises from the starting height (plate curvature) and the spring rate variation. As noted, spring rate depends on plate thickness and plate material. Thus, load variation can result from variation in the thickness or material composition of the load plate and its response to the application process (e.g., heat-treating and tempering).

[0028] The load variation from both of these variables can be eliminated by appropriately adjusting the starting height or curvature of the load plate so that a target load is achieved at the installed or deflected height of the load plate. Variation in material composition, material thickness and response to the fabrication process can be introduced with each new lot of load plate material, e.g., stainless steel, that comes into the load plate manufacturing process. Typically, the variation in material composition and thickness within a single lot is negligible.

[0029]FIG. 4 illustrates an exemplary method that can be use under the principles disclosed herein to eliminate these variables from the applied load of the plate when the load plate is placed in service. As shown in FIG. 4, the method includes adjusting the starting height or curvature of the load plate to produce the desired working load.

[0030] With each new lot of material (determination 130), a sample run of the load plates is made and measured (step 131). The starting height (S, FIG. 3a) or curvature of the plate and any change in height as the plate goes through the heat treating and sizing processes is tracked (step 132).

[0031] Then, measuring the deflection required to achieve the target load a new starting height for this lot of material can be determined (step 133). The tooling that shapes the curvature in the fabrication process is designed to be adjustable and the starting height is adjusted to match the material used and the desired working load.

[0032] A full production run can then be initiation (134). Consequently, the fabrication method is adjusted to minimize any variation in the working characteristics of the load plates produced from different material lots.

[0033] Previously, variation in the working load of load plates produced in different production runs has been significant and has decreased product consistency and reliability. The load tolerance due just to the standard thickness variation of +/−0.002″ is +/−41.5 lbs at a target load of 330 lbs or about +/−12% of the load. Further, the starting height of the finished plate varies by as much as 0.025″ as a new lot of material responds to the heat treating and sizing. At a spring rate of around 1700 lbs/in, the resulting load variation is +/−21 lbs.

[0034] Most of the combined +/−62 lbs tolerance in the final load can be eliminated by adjusting the part height to match the material properties as outlined in this disclosure. Consequently, load plates produced according to the methods described herein will promote product consistency and reliability.

[0035] The preceding description has been presented only to illustrate and describe embodiments of invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the following claims. 

What is claimed is:
 1. A method of making a load plate for an attach hardware assembly of a circuit assembly, said method comprising adjusting a curvature of said load plate based on a target load to be applied by said attach hardware.
 2. The method of claim 1, wherein adjusting said curvature of said load plate based on said target load, further comprises adjusting said curvature based on a material thickness and material composition of a material from which said load plate is being made.
 3. The method of claim 2, further comprising using stainless steel as said material.
 4. The method of claim 1, further comprising, for each new material lot being used to make load plates, performing a test run of load plate production.
 5. The method of claim 4, further comprising: measuring a starting height of at least one load plate of said test run as a measure of curvature; and tracking changes to said starting height through fabrication of said at least one load plate.
 6. The method of claim 5, wherein said fabrication comprises heat-treating the load plate.
 7. The method of claim 5, further comprising adjusting an original starting height for production of load plates based on said changes tracked in said test run and said target load.
 8. The method of claim 7, further comprising initiating a,full production run of load plates to use up said material lot.
 9. A method of making a circuit assembly, said method comprising: making an electrical connection between a circuit to a circuit board; determining a target value for a load to be applied with an attach hardware assembly to secure the connection between said circuit and circuit board; adjusting a curvature of a load plate of said attach hardware assembly based on a target load to be applied by said attach hardware.
 10. The method of claim 9, wherein adjusting said curvature of said load plate based on said target load, further comprises adjusting said curvature based on a material thickness and material composition of a material from which said load plate is being made.
 11. The method of claim 10, further comprising using stainless steel as said material.
 12. The method of claim 9, further comprising applying a load with said attach hardware assembly to secure the connection between said circuit and said circuit board.
 13. The method of claim 9, wherein said making an electrical connection comprises connecting an Application Specific Integrated Circuit in a socket of a circuit board.
 14. The method of claim 9, further comprising, for each new material lot being used to make load plates, performing a test run of load plate production.
 15. The method of claim 14, further comprising: measuring a starting height of at least one load plate of said test run as a measure of curvature; and tracking changes to said starting height through fabrication of said at least one load plate.
 16. The method of claim 15, further comprising adjusting an original starting height for production of load plates based on said changes tracked in said test run and said target load.
 17. The method of claim 16, further comprising initiating a full production run of load plates to use up said material lot.
 18. A system for making a load plate for an attach hardware assembly of a circuit assembly, said system comprising: means for sizing and shaping a load plate; and means for adjusting a curvature of said load plate as formed by said means for sizing and shaping said load plate, wherein said curvature is adjusted based on a target load to be applied by said attach hardware.
 19. The system of claim 18, wherein adjusting said curvature of said load plate based on said target load, further comprises adjusting said curvature based on a material thickness and material composition of a material from which said load plate is being made.
 20. The system of claim 18, wherein said load plate is formed comprising stainless steel.
 21. The system of claim 18, further comprising: means for measuring a starting height of at least one load plate of a test run; and means for racking changes to said starting height through fabrication of said at least one load plate.
 22. The system of claim 21, wherein said fabrication comprises heat-treating the load plate.
 23. The system of claim 22, further comprising means for adjusting an original starting height for production of load plates based on said changes tracked in said test run and said target load. 